The polarization of light is described by specifying the orientation of the wave's electric field at a point in space over one period of the oscillation. Polarization transformation is used in many optical devices, including, but not limited to, liquid crystal displays (LCDs), optical storage (e.g., CD/DVD/Blu-ray), 3D movie cinemas, optical remote sensing, and optical fiber networks. Polarization transformation that can be precisely controlled for incident light over a broad range of wavelengths is referred to as broadband (or achromatic) polarization transformation, and can be used in applications that involve human perception or multiple simultaneous channels at different wavelengths.
Polarization is a non-scalar quantity of light, which may be fully-, partially-, or un-polarized. One of way of describing polarization is the Stokes vector, which describes the possible polarization states as four intensities: S=[S0 S1 S2 S3[T. Optical elements that transform polarization can be described with 16 parameters that are most often arranged into a 4×4 Mueller matrix M. Accordingly, an input polarization SIN can be transformed into an output polarization SOUT by the following relation: SOUT=M·SIN. For almost all birefringent components used in this context, many of the elements of the matrix M can vary strongly with wavelength, which can make broadband polarization transformation challenging. Some examples of elements that provide polarization transformation are quarterwave retardation elements (which can be used to transform light having linear polarization to circular polarization, or vice versa), and halfwave retardation elements (which can transform light having linear polarization to a different linear polarization direction, or vice versa).
Narrowband (or strongly chromatic) polarization transformation can be achieved with homogeneous retarders with uniaxial birefringence, typically called waveplates. These waveplates have phase retardation that varies strongly with wavelength (i.e., Γ=2π(ne−no)d/λ=2πΔnd/λ, where Δn=(ne−no) is the birefringence), and an optical axis along the extraordinary index direction that does not vary strongly with wavelength. Waveplates can be formed with a wide variety of materials, including but not limited to birefringent crystals, stretched polymer films, and liquid crystal layers.
Broadband polarization transformation can be accomplished by combining at least two waveplates formed from different materials, in such a way that their fast and slow optical axes are opposed. This approach may rely on having an appropriate difference in the dispersion of the materials' birefringence. For example, crystal quartz and magnesium fluoride waveplates can be used for broadband polarization transformation. However, the availability of such natural minerals or grown crystals, as well as the size and cost of such elements, can be prohibitive in many cases, among other limitations related to performance.
Alternatively, broadband polarization can be accomplished using two or more discrete waveplates, typically formed of the same material, where optical axis orientations and individual retardations of the waveplates are not usually orthogonal. Some examples of this technique involve three waveplates, but it is also possible to implement embodiments with two, five, six, or more waveplates. While these waveplates can be formed with many types of available birefringent films (such as those mentioned above for narrowband waveplates, including liquid crystal layers), it may be necessary to form each discrete waveplate on its own as a physically separate element, and then subsequently assemble each separate element with a high level of precision relative to the other elements. This approach can substantially add to fabrication costs, can often lead to thick (i.e., many mm or cm) components, and can resulted in a constrained angular aperture, among other limitations.
An additional category of broadband polarization transforming elements includes single inhomogeneous birefringent layers, typically formed with uniaxially birefringent materials that have a local optical axis that is not uniform throughout the thickness. These birefringent layers have been used in LCDs and other optical devices. These birefringent layers can form the addressable layer, such as the 90° twisted nematic (TN) and super-twisted nematic (STN) LCDs, as well as compensation films with positive and negative birefringence. While these birefringent layers can act as polarization transformation elements, often with some achromatic behavior, they can have limitations with regard to the types of input and output polarizations that may be transformed. For example, 90°-TN and STN birefringent layers may only transform linear to near-linear polarizations, and many compensation films may make only small adjustments to the polarization. Also, a single twist layer can be used as a retarder to partially convert the circular polarization to linear polarization (for a single wavelength) over a relatively narrow bandwidth range. While this single twist layer may be combined with a cholesteric polarizer, these elements may be formed separately and subsequently assembled with each other, similarly resulting in problems with fabrication costs, thickness, performance, etc.
Combinations of twisted layers have also been used for broadband polarization transformation. For example, U.S. Pat. No. 6,765,635 describes two 135° twisted nematic layers on either side of a uniaxial halfwave layer can be employed as an electrically controlled polarization modulator. In another case, a broadband quarterwave retarder is provided using two twisted nematic cells, fabricated separately and subsequently assembled.